A high resolution temporal study of phytoplankton bloom dynamics in the eutrophic Taw Estuary (SW England)

Abstract

The Taw Estuary (SW England) is eutrophic as a result of enhanced nutrient inputs from its catchment. However, factors influencing the timing and extent of phytoplankton bloom formation are not fully understood in this system. In this study, high resolution chemical and biological sampling was undertaken in late-winter/spring and summer 2008 in order to gain further insights into bloom dynamics in the Taw Estuary. Temporal variations in chlorophyll a maxima in the upper and middle estuary during summer were controlled by river flow and tidal amplitude, with nutrient limitation probably less important. Concentrations of chlorophyll a were highest during low river flow and neap tides. Increased river flows advected the chlorophyll maximum to the outer estuary, and under highest river discharges, chlorophyll a concentrations were further reduced. This feature was even more pronounced when spring tides coincided with high flows. The main bloom species were the diatoms Asterionellopsis glacialis and Thalassiosira guillardii. Using two multivariate statistical techniques in combination, five distinct physical and biogeochemical states in the Taw estuarine waters were identified. These states can be summarised as: A(1), high chlorophyll a, high temperature, long residence times, nutrient depletion; A(2), strong coastal water influence; B(1), decreasing chlorophyll a, increasing river flow and/or spring tides; B(2), transitional between states A(1) and B(3); B(3), high river flow. It was thus possible to differentiate between contrasting environmental conditions that were either beneficial or detrimental for the development of algal blooms. A conceptual model of diatom - dominated primary production for the Taw Estuary is proposed which describes how physical controls (river flow, tidal state) moderate plankton biomass production in the upper and mid - estuarine regions.